Properties And Microstructural Controlling Of 1000MPa Grade Ultra-high Strength Steels During Rolling And Cooling Process

With the development of society,the demand of materials is increasingly high,especially the high strength structural materials.New requirements of steels,which possess high yield strength and are easy to weld because of the repaid development of construction machinery,oil exploitation,bridge construction,etc.High strength structural parts are subjected to multiple loads in service,so it will bring incalculable losses once the materials yield.Therefore,the research on investigating high strength high toughness steels to replace the low strength heavy plates and ensure the reliability of the structures is significant.In this paper,two kinds of Cr-Mo high strength steels were designed and conducted processing simulation and hot rolling.The microstructure evolution,mechanical properties,characteristics of simulated welding,gas shielded welding and hydrogen permeation properties were investigated.The main research work and the innovative results are as follows:(1)The effects of process parameters on the microstructure evolution and variation in macro-hardness of Nb-V-Ti precipitation strengthening low carbon Cr-Mo ultra-high strength excellent toughness steels were systematically studied.The finish rolling temperature was determined between 880 and 900 ? and the isothermal temperature was controlled at 400 ?approximately,the microstructure would completely transformed to bainite with refined laths.The transformation mechanism and mechanical properties of two process parameters were analyzed and compared during the actual hot rolling experiments.The results showed that the coiling and cooling parameters effectively affected the obdurability of Cr-Mo and Nb-V-Ti composite precipitated high strength steels.The yield strength of experimental steels was in the range from 1060 to 1145 MPa,and the elongation was in the range from 8.6 to 10.1%.The V-notch impact toughness value at-20 ? was in the range from 85 to 120 J/cm2.(2)The chemical composition,combined with Cr and Mo element,was designed with low carbon high silicon and without precipitated elements.By using refined microstructure and elements solution to enhance the strength,and film-like retained austenite to increase the ductility and impact toughness,ultra-high strength and excellent toughness steel was obtained.The isothermal transformation was conducted on the simulated testing machine,and the effects of austenitizing temperature,isothermal temperature and isothermal time were investigated,providing the theoretical basis for actual hot rolling experiments.(3)The research on the hot rolling experiments of low carbon high silicon Cr-Mo ultra-high strength steel indicated that refined carbide-free bainite and low temperature tempering dislocation martensite were obtained by using two stage hot rolling process and ultra-fast cooling.The width of bainite lath and film-like retained austenite was 0.1?0.5 ?m and 0.08?0.2 ?m,respectively.The yield strength,tensile strength,and elongation were in the range from 1010 to 1070 MPa,1160 to 1310 MPa,12.3 to 20.8%,respectively.The impact toughness value at-20 ?was in the range from 88 to 115 J/cm.In addition,the on line heat treatment process was investigated.The results showed that low temperature tempering can reduce the residual stress,and slightly affect the strength,ductility and impact toughness,nevertheless,the high temperature tempering dramatically improves toughness and decreases the strength which cannot satisfy the mechanical property requirements.(4)The influence of heat input on single pass coarse grained heat-affected zone and the effects of second peak temperature on double passes reheated coarse grained heat-affected zone were investigated on the simulated testing machine.The microstructure of single pass coarse grained heat-affected zone changed from lath martensite to lath bainite,then to coarser granular bainte.By increasing the heat input,the impact toughness rised at first and went down latter.The main reason is that martensitic fraction is dominant in the martensite-austensite consitituents resulted by carbon fully diffusion,and brittle fracture by concentrated stress easily occurs.The impact toughness of double passes simulated reheated coarse grained heat-affected zone slightly decreased,which showed the impact toughness deterioration of high strength high toughness steel by multi-pass welding will not appear.(5)The actual Ar+20%CO2 gas shielded welding experiment showed that the evolution of microstructure and the variation of strength and low temperature impact toughness were dramatically influenced by heat input energy.The strength of welded joints decreased as the heat input increased.Once the heat input increased to 20kJ/cm,the welded joints were intenerated and the polygonal ferrite appeared.With reasonable welding heat input energy,the process of eliminating hydrogen for tempering one hour at 200? and interpass temperature not lower than 150 ?,the welding cold cracks did not appear without preheating before welding.The yield strength,tensile strength,elongation and the impact absorb energy at-20 ? of welded joints were 1005 MPa,1120 MPa,11.5%,and 55 J,respectively.(6)The hydrogen permeation properties of experimental steels with different chemical composition,different hot rolling and heat treatment process were investigated.The hydrogen permeation current was measured by Devanathan electrochemical law with cathode electrolysis constant current.In addition,hydrogen permeation flux,the hydrogen concentration of hydrogen trapping,and hydrogen diffusion in different process conditions were calculated.The results indicated that the hydrogen diffusion coefficient in carbide-free bainite specimens was higher than that in dislocation martensite specimens.The main reason was that the hydrpgen absorption capacity of retained austenite and the interfaces of austenite-martensitic lath were stronger than that of dislocations.The hydrogen diffusivity of dislocation martensite was low since high hydrogen trapping coefficient.After low temperature tempering,the density of mobile dislocations decreased,the yield strength and hydrogen diffusion coefficient of experimental steels increased.The research of comparing the hydrogen permeation properties of 2Mn,3Mn,and 3Mn-0.5Ni experimental steels indicated that hydrogen diffusion coefficient was not only influenced by interfaces due to grain refinement,but also the hydrogen diffusion activation energy,which caused by lattice distortion of solid solution atoms.The effect of Mn and Ni element was on the contrary,namely,Mn hindered the hydrogen diffusion while Ni accelerated the hydrogen diffusivity.